Normally a piece of furniture having a upward projection like an arm or back rest is typically constructed by building a normally wooden frame and then securing the necessary springs, padding, and coverings to this frame. Even though the materials applied to the frame do indeed in part mask the shape of the frame, its shape nonetheless largely dictates the general massing of the finished furniture.
The frame itself must be very robust and relatively accurately dimensioned. This is difficult to do for other than a skilled craftsman with wood or metal, and makes it impossible to substantially change the shape of a piece of furniture as it is being built and designed.